This type of issue can be because of an obstructed vent preventing the air to easily flow to the outside. A quick way to test this theory is to unhook the dryer vent and run the dryer (if it is a gas dryer be sure to make sure you have adequate ventilation in the room where the dryer is located. This should only be done to see if the dryer performs better and see if the top of the dryer is just warm like it should be, not hot.
If the timer advances and the top is not as hot as it was then you know the venting must be the issue.
Hope this helps!